This invention relates to a solar energy system and particularly to a unique collector apparatus having an integrated means for controlling of the temperature of the collector and thereby preventing damaging high temperature conditions in the collector and the system.
Solar collector apparatus to collect and transmit the energy of the sun for useful purposes has been suggested for many years. With the recent increases in the cost of energy and the realization of the finite nature of natural energy resources, interest in the practical collection and use of solar energy has significantly increased. Generally, solar energy transfer systems employ an external collector means exposed to the solar radiation in combination with a circulating transfer means for circulating a transfer medium, such as a suitable liquid, and thereby transferring of solar energy from the collector to a storage means. A conventional and relatively well-developed collector unit or apparatus is the flat plate solar collector in which a pair of parallel plates define an energy entrapment source or chamber, with the inner plate including a suitable absorber. A transfer mechanism is coupled to the inner collector plate for transfer of the trapped solar energy from the collector. Generally, the inner plate has a suitable coated surface for increasing the energy absorption efficiency. The system may operate with periods of relatively low liquid flow during periods of low load demand and even periods of non-circulation and high insolation if the storage unit becomes fully charged. During such periods, the absorber surface continues to absorb the solar energy and the surface may reach exceedingly high temperatures above ambient. For example, a temperature of 400.degree. F. above the ambient temperature is possible under long periods of high insolation. Such temperatures can rapidly cause degradation in the coating on the absorber and even cause sufficient thermal stress within the collector resulting in breakage of the glass covered assembly. As such problems are well-known, various protective methods have been proposed. The collector flow system may, for example, be provided with a vent means for venting of steam which is generated in the collector transfer tubing. Although this results in a reduction in the temperature, make-up liquid must be provided upon reestablishment of normal system operation. This requires the necessary complex make-up source and controls as well as increasing the cost of providing of appropriate liquid. An alternative means which minimizes the liquid loss includes provision of an expansion tank connected to the collector system through a suitable pressure release connection. The tank serves as a reserve storage element for any expanded vapor or liquid under increased temperature conditions. Although it prevents excessive loss of collector fluid, it also requires additional components and controls and may not efficiently maintain the temperature at a desired level. Other systems have suggested suppression of vapor formation by creating of a high pressure flow system. This system may also employ an expansion tank with regulated gas pressure applied to the top of the expansion tank thereby maintaining the desired high pressure operation. Although the high temperature condition may be minimized, the apparatus obviously requires a relatively complex overall system.
Although the prior art thus includes various means for minimizing the creation of damaging high temperature conditions in the solar collector, such systems generally are based upon dissipation of collected energy with additional controlled components. There is a need for a relatively simple and reliable control for solar collectors and particularly flat plate solar collectors.